On the Ordering Mechanism of Cu ⁺ in 2D van der Waals Multiferroic CuCrP ₂ S ₆

CuCrP₂S₆ is a van der Waals multiferroic where the tunable Cu⁺ sublattice underpins its exceptional ferroelectric and electronic switching properties. Yet, the microscopic mechanism governing Cu⁺ ordering has remained elusive. Here, we combine single-crystal X-ray and neutron diffraction with pair distribution function analysis to uncover a temperature-driven evolution of Cu⁺ ordering, giving rise to an incommensurate quasi-antipolar phase between the paraelectric and antiferroelectric states. The modulation originates from correlated Cu⁺ occupancy redistribution coupled to breathing distortion of surrounding S₃ triangles, establishing a symmetry-adapted lattice distortion mode. Diffuse scattering persisting over 35 K above the transition confirms that the structural instability follows an order-disorder mechanism. The spontaneous off-centering of Cu⁺ positions CuCrP₂S₆ as a model platform for correlated order-disorder phenomena in 2D layered ferroics, and provides design principles for next-generation memory and logic devices.